A-staged thermoplastic-polyimide (TPI) adhesive compound and method of use

ABSTRACT

A compound and method of use thereof consisting of an A-staged thermoplastic-polyimide (TPI) adhesive, a viscous uncured liquid of polyamic-acid polymer (PAA), the TPI precursor, synthesized and dissolved in a polar aprotic organic solvent, and including, as appropriate, combinations of particulate ceramic and/or metallic thermally conducting, electrically insulating, and thermally conducting, electrically conducting fillers for interface-bonding to create a robust joint between surfaces with conventional lamination processes that utilize relatively moderate temperatures and applied pressures.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on a provisional application Ser. No.62/123,850 filed Dec. 1, 2014 by the same inventor and is herebyincorporated by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to thermoplastic adhesives and more particularlyto thermoplastic-polyimide (TPI) A-staged adhesives.

Description of the Prior Art

The employment of partially cured (B-stage) or fully cured (C-stage) TPIcompounds disposed on substrates as adhesives are known in the priorart. However, as best known to the inventor, it is unknown to use theprecursor of these compounds in an uncured (A-stage) state, either withor without suitable fillers for bonding purposes by applying the uncuredTPI precursor in liquid form (A-stage) directly to the surfaces to bejoined and thereafter curing entirely In Situ or at the site.

As described below the use of the A-staged TPI adhesive of the inventionmay be particularly useful in, for example, the fields of lamination ofa semiconductor die to an aluminum heat sink (die-attachment) forthermal management of high-power electronic packaging and, additionally,the bonding of superconducting coils.

Die-attachment on CTE Mismatched Heat Sink for Thermal Management

A lamination of a semiconductor die to an aluminum heat sink is commonin high-power applications, as the heat sink dissipates the heatgenerated from the semiconductor. As the laminated materials typicallyhave severely mismatched coefficients of thermal expansion (CTE), forexample, aluminum (˜23 ppm/° C.) and semiconductor (˜3-8 ppm/° C.), thebondline between the die and the heat sink undergoes significantinterlaminar stress during the temperature excursions of processing anduse. The failure of the bondline between the semiconductor and the heatsink will dramatically reduce thermal transfer between the surfaces,leading to overheating and failure of the semiconductor.

For simplicity, only die-attachment directly to an aluminum heat sink isdiscussed. This TPI technology also applies to bondlines of otherinterlayers between the semiconductor and aluminum, which are commonlyused to stepdown CTE mismatch between the semiconductor and the aluminumheat sink. These interlayers can provide submount substrates for thesemiconductor die or dice, as well as a CTE buffer between thesemiconductor and the aluminum heat sink. Interlayers could includeceramics such as alumina, Al₂O₃, CTE of 7 ppm/° C., and aluminum nitride(AlN), CTE of 4.6, among others; metals such as copper, CTE of 17, amongothers; printed circuit boards such as FR4 glass/epoxy, CTE of 12, amongothers; and various composite materials such as AluminumSiliconCarbide(AlSiC), CTE of 7-11, among others.

Conventional die-attachment is often done with thermoset epoxy polymersthat have been filled with metal powder to enhance its thermalconductivity. Often, electrical conductivity of the die-attach bondlineis also critical. These thermoset epoxy polymers are brittle (bothunfilled and especially filled), and so the die-attach epoxy bondline isdesigned to withstand the inter-laminar stress without inducing cracksin the epoxy, which would propagate with time and temperature cycling.Reducing the thickness of the bondline would exponentially increase theinter-laminar stress between the die and heat sink, and so epoxybondlines have a minimum dimension of 0.7-1.5 mil (18-35 um).

To maximize thermal transfer and potentially electrical transfer betweenthe die and heat sink, highly conductive metal powders, such as silveror copper, are compounded into A-staged epoxy resin. The concentrationof these metal powders can reach 80% by weight or more, as solids in thecured bondline. As silver is a precious metal, and is often used in acostly micro-sized or even nano-sized format, the cost component of themetal in the bondline is significant, especially when the bondline needsto have a thickness of 1-mil (25 um) or more.

When thermal conductivity, but not electrical conductivity, of thedie-attach bondline is desired, ceramic powders are used as fillers inepoxy bondlines. Ceramic powders, such as alumina and boron nitride, arehigh thermal-conductivity dielectrics.

In processing, the die-attach epoxy is applied to the heat sink surfacemanually or with an automated dispenser. The semiconductor die is thenprecisely placed onto the epoxy. The subassembly's bondline is thencured with heat, in a controlled manner that allows outgassing andavoids voiding. Some pressure may be applied.

The use of A-staged TPI polymer has the following advantages indie-attach over the epoxy technology described above:

TPI polymer will not crack, allowing much thinner bondlines between CTEmismatched surfaces and potentially enabling higher loadings of metalparticles, which would further embrittle the already brittle curedepoxy.

Thinner die-attach bondlines will enable higher thermal and electricaltransfer between the die and the heat sink.

Thinner die-attach bondlines will utilize much less material, providingsubstantial cost savings.

While epoxy die-attach bondlines have a maximum temperature rating of175° C. or less, TPI bondlines can operate continuously at well above300° C. This will become increasingly important with the transition towide band-gap semiconductors, such as SiC and GaN, which can operatevery efficiently at high temperature.

Die-attachment can also be done with eutectic solders, in pre-forms oras paste, compounded with an organic flux that prevents oxidation of thesurfaces at high temperature and promotes surface wet-out, ensuring anoptimal bondline. The solder die-attach is very electrically andthermally conductive, and provides a robust ductile bondline thatprovides a buffer between CTE-mismatched surfaces.

These high-performance solders are generally made with precious metals,such as silver (Ag) and gold (Au), and require extreme reflowtemperatures, such as 363° C. for AuSi, for die-attachment. Theprecious-metal solders generally have bondline thicknesses in the 1-10mil (25-250 um) range. As aluminum heat sinks do not provide a readilysolderable surface, the targeted aluminum area requires a metal platingor braising of a precious metal to ensure a robust solder joint betweenthe semiconductor die and the heat sink. This primer metallization isgenerally 0.08-0.15 mil (2-4 um). Both the raw materials and requiredprocesses for eutectic solder die-attachment are very costly.

In addition, sintered-silver technology is now used widely fordie-attach. Micro- and nano-sized silver particles are used in arelatively thick bondline, generally 50 um or more, often with apolymeric binder. Due to the extremely small size of the silverparticles, they will sinter to adjacent particles at processtemperatures (200-300° C.) well below the melting temperature of silver(962° C.), and form a robust, relatively ductile bondline between thedie and heat sink. High pressure applied to the die during sinteringlamination is often required. Priming the die and/or heat sink isrequired.

In contrast to the prior art using precious-metal bondlines, the use ofA-staged TPI polymer has the following advantages in die-attachoperations over precious metal solder technology described above:

The material cost of the TPI polymer is much lower than precious-metalsolder;

The equipment requirements and process cost of the TPI bondline is lowerthan the process cost of precious metal solder or sintering;

Much lower temperatures are utilized in curing the TPI than in reflowingthe precious-metal solder;

Lower temperature and/or pressure is required for TPI lamination, whichalleviates applied stresses on the semiconductor, and

TPI generally does not require a prime coat to bond to aluminum. Theprecious-metal plating or braising of the aluminum surface to be bondedis expensive in both material and process cost. When priming of a metalsurface is required to ensure a robust bondline, a simple wipe with theA-staged TPI liquid and then quick bake to drive off the solvent andB-stage the polymer suffices.

Superconducting Magnet Coil Insulation and Bonding

Superconducting magnet coils operate at cryogenic temperatures, generatevery high stress within their structure due to the required temperatureexcursions, and often need be highly radiation-resistant, due to theirapplication environment.

Polyimide polymers, both in film form, for example, DuPont Kapton® andKaneka Apical®, and in TPI C-stage adhesive form have long been used forsuperconducting magnet coil insulation and bonding, as polyimides haveamong the highest radiation-resistance of any polymer. Superconductorsare made with metal alloys that are reacted at very high temperature, ashigh as 900° C., to provide their superconductivity. These processtemperatures would, of course, destroy any organic-polymeric components.

In react-and-wind superconductors, such as Niobium-Titanium (NbTi), thesuperconductor wire is ductile and can be handled like an ordinarycopper wire after its high-temperature reaction processing. Therefore,its cable form can be wrapped with polyimide film. This polyimideinsulation would have an adhesive coating, such as epoxy. The cable canthen be wound into the desired coil and bonded into precise shape with ahigh-pressure, elevated-temperature lamination.

These NbTi magnets are by far the most common in today's particleaccelerators, such as CERN's Large Hadron Collider and BrookhavenNational Lab's Relativistic Heavy-Ion Collider. However, to obtainhigher magnet fields for specific experiments on these rings, asuperconductor that can carry much more electric current than the NbTicables is required, i.e., Niobium-Tin (Nb₃Sn).

Nb₃Sn is a wind-and-react superconductor. After itsultra-high-temperature reaction, it becomes extremely brittle and cannotbe bent or wound, as it would readily crack. Therefore, the desiredNb₃Sn coil must be pre-wound before the reaction process, with glassfabric separating the individual conductors as glass can survive thehigh-temperature reaction process.

After the reaction, the now-superconducting Nb₃Sn coil is impregnatedwith an A-staged liquid polymer that serves as both an insulation and abonding agent. Thermoset epoxy, such as CTD 101 resin from CompositeTechnologies Development, Inc of Lafayette, Colo., has served as thebaseline coil-impregnation; thermoset polyimide, such as Matrimid resinfrom Huntsman Corporation of Houston, Tex., has also been considered,due to its higher radiation-resistance.

These thermosets are all very brittle, and crack with the inevitabledownstream handling and operation, reducing the glass plus thermosetinsulation layer to only about 100 V/mil, which is the dielectricstrength of air. In addition, the ‘cracking nature’ of the thermosetscauses quenching issues with the assembled coils, which will shiftslightly when reaching an increased level of power, causing a crack inthe brittle thermoset installation, releasing enough energy to eliminatesuperconductivity in the adjacent Nb₃Sn cables, i.e., a quench. Thisarea quickly becomes a hot spot in the coil, and the magnet must be shutdown and restarted, which is a laborious and time-consuming process. Anindividual superconducting magnet needs to reach a specific power level,therefore multiple quenches of often as many as 20-50 cycles aresometimes required. This process is called “training” the magnet.

In contrast to the prior art, TPI polymers are ductile andcrack-resistant, even at cryogenic temperatures, and, as such, Nb₃Snsuperconducting coils that are vacuum-impregnated with an A-staged TPIsolution, which is then dried and cured at high temperature, will notexhibit significant loss-of-dielectric properties, nor require extensivetraining to reach their required power levels. The impregnation of thecoils can be assisted by heating the A-staged TPI solution whichdramatically decreases its viscosity. The polymer in the A-staged TPIsolution is stable even at impregnation temperatures up to 200° C.

Superconducting magnet bondlines made with insulation impregnation ofA-staged TPI solution will have significant performance advantages overconventional thermoset dielectric/adhesive systems including:

The structural and dielectric integrity of the coil will be much higher;

No cracking will occur and, therefore considerable less training will berequired; and

Higher radiation resistance will be obtained.

SUMMARY OF THE INVENTION

The invention may be summarized as both the product and the use of anA-staged thermoplastic-polyimide (TPI) adhesive, a viscous liquid withpolyamic-acid polymer (PAA), the TPI precursor, synthesized anddissolved in a polar aprotic organic solvent for interface-bonding tocreate a robust joint between surfaces with conventional laminationprocesses that utilize relatively moderate temperature and appliedpressure. Utilizing A-staged TPI adhesive maximizes the polymer's flowand reactivity allowing lamination of surfaces at temperatures andpressures well below what is required to bond that same TPI layer whenapproaching a fully cured state. The minimized temperature and pressurerequired for bonding dramatically reduces the required processingconditions and equipment, as well as the stress applied to the laminatedsurfaces. Moderated process conditions are critical in manyapplications, for instance, the attachment of semiconductor die andpower devices to heat sinks described above, to eliminate the hightemperatures and pressures that could which damage delicatesemiconductor components.

The cured thermoplastic polyimide (TPI) bondline is very thin anddurable in tensile and shear strength, across a wide range of conditionsand exposures, as opposed to those of thermosets, which are bycomparison quite brittle. This is especially true even with laminates ofdissimilar materials and mismatched coefficients-of-thermal-expansion(CTE). For example, the TPI bondline can withstand harsh thermal shocksup to a temperature variance of 400° C. and extreme temperatureexposure, without structural or dielectric degradation. For example,extreme temperature exposure can include cryogenic temperature down toliquid Helium, −269° C., greater than 300° C. continuously and greaterthan 400° C. for short periods of time.

This relative thinness also allows the optimization of thermal andelectrical conductivity across the bondline, minimizing material cost,an extremely important factor for coatings with precious-metalcompounding.

Additionally, TPI bondlines have a wide range of useful applicationsranging from electronic packaging and superconducting magnets, tojewelry and art. For example, TPI can robustly bond ceramic andglass-like materials to metal surfaces.

In addition, TPI polyimide structure provides excellent chemicalresistance.

The following are relevant characteristics and properties of theinvention:

A-staged TPI adhesive utilizes polyamic-acid (PAA) polymer synthesizedand dissolved in a low-solids solution with polar aprotic solvent. PAAis the precursor to thermoplastic polyimide (TPI) polymer.

A-staged TPI adhesive is a one-part system. As opposed to the two-parthigh-performance epoxy adhesives, A-staged TPI adhesive is very stable.At room temperature, its pot-life is measured in days; when kept in afreezer, its shelf-life is indefinite.

The liquid A-staged TPI adhesive can be compounded with fillers totailor the properties of the bondline.

The viscosity of A-staged TPI adhesive is highly dependent ontemperature. This characteristic could be helpful in the dispensing ofthe material onto surfaces.

The PAA polymer dissolved in the A-staged TPI adhesive is stable atsolution temperatures of up to 200° C. for short periods.

A-staged TPI adhesive can be applied to a bond joint (2-25 um, dry),even between materials with dissimilar CTEs.

A coating of A-staged TPI can be partially cured (B-staged) with heat ina bondline, evaporating the majority of the solvent and converting someof the PAA to TPI.

Solvent activity in the bondline can be beneficial in ensuring wettingwith the laminating surfaces. Micro-scouring of the bondline surfacesmay also be beneficial. Evaporating solvent can assist the removal ofair from a TPI bondline.

When the solvent is largely removed, the PAA polymer in the coatinggradually converts to TPI polymer, releasing water vapor, which shouldbe allowed to escape the bondline. It is critical to manage thisphenomenon to avoid blistering.

These and other characteristics and advantages of the invention will befurther understood from the description of the preferred embodiment inconjunction with the drawings which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of the chemical process of the invention;and

FIG. 2 is a graph relating two parameters of the operation of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

TPI coatings are made by polymerizing polyamic-acid (PAA) polymer inpolar aprotic solvents, such as NMP (N-methylpyrrolidone), DMAc(dimethylacetamide), and DMF (dimethylformamide). The PAA's solidsconcentration can be 5-40% in solution (by weight), and commonly 15-25%.TPI-PAA solutions are a one-part adhesive, and very stable when kept ina freezer or left out at room temperature for a few days.

Typical TPI diamine can be, for example, one or more of the followingmonomers: 3,5-diaminobenzoic acid (DABA), 3,3′-diaminobenzophenone(3,3′-DABP), 3,4′-diaminobenzophenone (3,4′-DABP), diester diamine(RDEDA), 1,3-bis-(4-aminophenoxy) benzene (TPER), 3,4′-oxydianiline(3,4′-ODA), 4,4′-oxydianiline (4,4′-ODA), 4,4′-methylene dianiline(4,4′-MDA), an aliphatic diamine, or a silicone-diamine among others.

Typical TPI dianhydride can be one or more of the following monomers:3,3′, 4,4′-biphenyltetracarboxylic dianhydride (BPDA), 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA), 4,4′-oxydiphthalicanhydride (ODPA), pyromellitic dianhydride (PMDA), or2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA)among others. TPI-precursor solutions, polyamic-acid polymer insolution, are also available commercially, such as LARC-TPI orFraivillig Technologies FM901 solutions.

TPI coatings can be compounded with powder or particulate fillers suchas ceramic, metal and pigments to tailor the properties of the bondline.On a solids basis, fillers can be compounded from 5-98% (by weight) intothe TPI polymer. There are many fillers that could be used to optimizethe properties of a TPI bondline, but these examples will cover a largemajority of applications. Representative thermally conductive,electrically insulting inorganic fillers for loading A-staged (liquidprecursor) thermoplastic polyimide (TPI) include:

Boron nitride (BN) powder and flake, available from MomentivePerformance Materials Inc., Strongsville, Ohio;

Alumina fumed powder, available from Evonik Industries AG, Parsippany,N.J., and Cabot Corporation, Billerica, Mass.; and

Boron nitride (BN) nano-tubes, available from Tekna Advanced MaterialsInc., Sherbrooke, Quebec.

These fillers can be combined to optimize properties, such as BNplatelets (which are relatively large, a few microns) with fumed alumina(which is submicron), as this maximizes the amount of property changingceramic. Representative thermally conductive, electrically conductiveinorganic fillers include:

Silver (Ag) flake, available from Metalor Technologies SA, NorthAttleboro, Mass.

The TPI coating can be applied to surfaces to be bonded with a range ofconventional technologies, even a simple wipe. The viscosity of theTPI-PAA solution is very sensitive to temperature, yet stable, a featurewhich can be utilized in tailoring for a specific application of the TPIcoating.

Pre-treatment of the surfaces to be coated, such as corona, plasma, orflame treatment, may improve the wetting of the TPI coating and eventualadhesion of the cured TPI bondline, but is often not required.

The surfaces to be bonded are then assembled together, ensuringexcellent contact between them. Pressure can be applied mechanically toensure intimacy. It may be a goal to minimize applied pressure, as thatcan result in residual stress in the finished laminate.

The TPI coating is tacky as a liquid at room temperature before thedrying and curing process. As it dries, resulting in solventevaporation, the partially dried coating will be naturally tacky attemperatures above what was the previous maximum process temperate for ashort period until the solvent evaporates to its new equilibrium withinthe polymer matrix. This tacky feature may be advantageous in assemblyoperations.

Since liquid TPI coatings are relatively low-solids, typically 15-25%,the initial thickness of the bondline in processing will be much greaterthan the finished cured bondline. Using a TPI coating solids of 20%, thefinal TPI bondline would be less than 1/7^(th) the initial wetthickness. The final cured thickness of a TPI bondline can be 1-20 um.Assuming a solids-level of 20%, the initial A-staged bondline would beapproximately 7-140 um.

Heat is then applied to drive off the solvent and cure the TPI polymerin a bondline made with TPI coating. This process can be done withconventional ovens, vacuum ovens and hot plates.

Depending on the application, heat can be increased gradually over acontrolled cycle or can be applied quickly, such as when placing anassembly on a hot plate.

As the TPI coating within the bondline heats up, its viscosity dropssignificantly and the solvent begins to evaporate. These actions canfacilitate surface wetting of the laminate, which can optimize thefinished bondline for strength and intimacy. It is important to notethat the polar aprotic solvent has relatively low surface tension, whichfacilitates its evacuation from a bondline as a vapor withoutsignificant bubbling as opposed to water.

Before it evaporates, the activity of the aggressive polar aproticsolvent at elevated temperatures can be beneficial to the finalbondline, as the solvent scours the surfaces to be bonded.

As the TPI bondline approaches 100° C., the solvent begins to evaporateand evacuate the bondline. The effect escalates as the bondlinetemperature increases. During this time, the solvent vapor can purge thebondline of residual air.

When most of the solvent has evaporated, i.e., when the bondline is at180-200° C., the PAA polymer will start converting to TPI, which is acondensation reaction that evolves water vapor as shown in FIG. 1. Thiswater vapor will have a very high vapor pressure, which is considerablyhigher than applied pressure on the laminate, so the water will escapecleanly as shown in FIG. 2.

After the conversion to TPI, there will be no additional evolution ofwater, and the micro-channels from which the water vapor escaped willcollapse.

Maximum process temperature that the TPI bondline should see isdependent on the application. For moderate temperature applications, theprocess temperature should be 10-20° C. above the expected maximumdownstream temperature in manufacturing or use. For high temperatureapplications, such as 300° C. and above, the maximum process temperatureof the bondline should ensure that the TPI polymer is fully cured, as noadditional water would be evolved.

After the water outgassing, at or near the maximum process temperature,additional pressure can then be applied to ensure the adhesion andintimacy of the bondline. Duration of the pressure is not typically afactor with TPI bondlines, which is helpful in minimizing process time.

TPI bondline assembly can be assisted with vacuum lamination, whichhelps the removal of evaporating solvent and water evolved from thePAA's condensation reaction to PI.

An A-staged TPI coating in contact with an existing B-staged TPI surfacewill allow the B-staged coating to absorb a portion of the solvent inthe A-stage coating, which solidifies that bondline over time, if onlytemporarily, until full curing at high temperature. The samesolvent-absorption effect is seen with lesser B-staged TPI coating i.e.,less cure, more solvent, on greater B-staged TPI coating i.e., morecure, less solvent. This mating effect of surfaces with similarchemistry, but dissimilar phase states (A-stage vs. B-stage; lessB-stage vs. more B-stage) enables temporary mating of surfaces, withfull lamination at the final cure at higher temperatures.

As long as there is enough pressure to ensure contract between thelamination surfaces, then tooling and the applied pressure can beminimized during the lamination process. This ensures that minimalinternal stresses are inherent in the laminate when it cools from theprocess temperature. When the laminated assembly heats back up towardsits maximum process temperature during downstream processing andoperation the internal stresses will be reduced.

Assessing and monitoring the level of TPI cure can be critical to ensureproperties and avoid further polymer reaction from causing blistering,when the part sees elevated temperature. This is especially important inapplications where the expected temperature is above the final TPI-curetemperature. Cure level of the TPI polymer can be assessed accurately bymonitoring the electrical-resistivity (ion-viscosity) of the bondline;the precursor PAA polymer has a low resistivity; TPI has a highresistivity.

The TPI coating can be applied to one or both surfaces to be bonded. TPIcoating(s) can be partially cured or B-staged, which gives the coatingstability at room temperature and ensures consistent thickness with hightemperature lamination (greatly reduced squeeze-out with appliedpressure).

B-staged TPI adhesive coatings are stable at room temperature and havean indefinite shelf life. This facilitates the manufacturing and storageof TPI products and intermediate-process assemblies.

B-staged TPI adhesive coatings and bondlines may have residual solvent(10-50%), but will act as a solid at room temperature.

The effective glass-transition temperature (Tg) of B-staged TPI coatingsand bondlines is the highest temperature that that polymer hasexperienced in previous processing. Above this temperature, the B-stagedTPI will soften and become tacky again, which may assist assembly. Asfurther solvent is lost and additional PAA polymer converted to TPI, theeffective Tg of the B-staged TPI coatings and bondlines increases.

Surfaces to be bonded with TPI can be pre-primed with A-staged TPIadhesive which would then be B-staged, before being bonded by additionalA-staged TPI adhesive.

During high-temperature TPI lamination, it is critical that the surfacesare in intimate contact, as the bondlines are relatively thin (2-10 um,typically).

Pressure can be applied with hardware or platen. Less pressure locks inless inherent stress between the lamination layers. Even the laminationof surfaces with no applied pressure, i.e., just the force of gravity onthe stacked parts, can be an effective bondline. Assembly clips andother hardware can apply pressures of 1-50 psi during TPI lamination.This moderate pressure allows the solvent and evolved water vapor (whichhas a very high vapor-pressure at high-temperature TPI lamination) toevacuate the bondline.

The maximum TPI lamination curing process temperature is applicationdependent. If the dielectric properties of the TPI do not require highdielectric strength or resistivity (residual PAA is low in both, but hasgood structural properties), then a maximum temperature of 150-200° C.will suffice. If the dielectric properties are critical, then a highermaximum temperature of 200-300° C. is recommended. Maximum laminationtemperature should be 10-20° C. above the highest expected downstreamprocess or application temperature. If the expected downstream processor application temperature is extremely high (300-450° C.), then it iscritical that full curing of the TPI bondline is ensured, through bothprocess temperature and cure time. If the TPI is not fully cured, thenencountering higher temperatures will result in additional wateroutgassing from subsequent curing of PAA to TPI at very high vaporpressure, which results in blistering and delamination.

Dwell time will be application dependent. The PAA polymer cures fasterto TPI at elevated temperature.

Full curing of a TPI bondline can be determined with the polymer'selectrical-resistivity (ion-viscosity) measurement.

Accordingly, the invention described above is defined by the followingclaims.

What is claimed is:
 1. The process of interface bonding of two surfacesforming a bondline, said process comprising in combination: A. providingan adhesive solution comprising an A-staged uncuredthermoplastic-polyimide (TPI), said thermoplastic polyimide having thecharacteristic of being insoluble in an organic solvent in the fullyimidized, fully cured state, in the form of a viscous liquid solutioncontaining in combination:
 1. a quantity of polar aprotic organicsolvent;
 2. a quantity of TPI precursor polyamic-acid polymer (PAA)synthesized and dissolved in said solvent wherein said polyamic-acidpolymer comprises a mixture of diamine and dianhydride monomers, andwherein said diamine monomer is selected from the group consisting of3,3′-diaminobenzophenone (3,3′-DABP), 3,4′-diaminobenzophenone(3,4′-DABP), 1,3-Bis (4-aminophenoxy) benzene (TPER), 3,4′-Oxydianiline(3,4′-ODA), 4,4′-Oxydianiline (4,4′-ODA), 4,4′-Methylene dianiline(4,4′-MDA), an aliphatic diamine, and a silicon-diamine; and whereinsaid dianhydride monomer is selected from the group consisting of3,3′,4,4′-Biphenyltetracarboxylic dianhydride (BPDA),3,3′,4,4′-Benzophenone tetracarboxylic dianhydride (BTDA),4,4′-Oxydiphthalic anhydride (ODPA), Pyromellitic dianhydride (PMDA),and 2,2′-Bis-(3,4-Dicarboxyphenyl) hexafluoropropane dianhydride (6FDA);and
 3. a quantity of particulate filler, B. applying said uncuredsolution to at least one of said surfaces; C. applying pressure to saidbondline in a selected amount of between 0 and 100 psi; and D. applyingheat to said bondline at a selected temperature of between 150 and 470°C., thereby converting said PAA to TPI, in situ, to form said bond. 2.The process of interface bonding of claim 1 wherein said particulatefiller comprises a quantity of thermally conducting solid particulatefiller in the amount of between 5 and 98% by weight.
 3. The process ofinterface bonding of claim 1 wherein said particulate filler comprises aquantity of electrically conducting solid particulate filler in theamount of between 5 and 98% by weight.
 4. The process of interfacebonding of claim 1 wherein said particulate filler comprises a quantityof electrically insulating solid particulate filler in the amount ofbetween 5 and 98% by weight.